The term “valve metal” represents a group of metals including aluminum, tantalum, niobium, titanium, zirconium, etc., all of which form adherent, electrically insulating metal-oxide films upon anodic polarization in electrically conductive solutions. The performance of valve metal and other types of capacitors depends upon several factors (e.g., the effective surface area of the anodes and cathodes that can be contacted by electrolyte, the dielectric constant of the oxide formed on the metal surface, the thickness of the oxide layer on top of the metal surface, the conductivity of the electrolyte, etc.). The thickness of the anodic oxide layer is approximately proportional to the electrical potential applied to the anode during the formation of the anode (i.e., at the time when the anode is immersed into the formation electrolyte). For aluminum, the oxide grows approximately by ˜1.0 nm per Volt, for tantalum this “growth rate” is somewhat higher, approximately 1.7 nm per Volt. Niobium and tantalum anodes are typically made in the form of a pressed powder pellet or “slug” when used in an electrolytic capacitor.
The density of the pressed anode slug is typically significantly less than the density of the bulk metal of which the powder is made, i.e., up to ⅔ of the volume of a given slug may be open space (pore space). The final density of the anode slug is largely determined at the time of pressing, when a known amount of powder is pressed into a known volume. Traditionally, formation of the anode slug has been thought to require a fairly homogeneous distribution of open space throughout the anode slug since the forming electrolyte needs to wet even the most “remote” cavities in the karst-like internal structure of the anode. This is specifically important for comparatively large anodes with volumes of the order 1 cm3 or above.
Furthermore, free flow of liquid electrolyte, both during initial surface processing (e.g., formation of surface oxide on the anode, also referred to as anodization) and operation as an electrochemical cell, continues to dominate capacitor design. One reason relates to the fact that the electrolyte used during anodization, typically referred to as a “formation electrolyte,” can become overheated within the interstices of the anode. This overheating adversely affects oxide formation and may cause electrolyte residue to accumulate, further compromising performance of the completed electrochemical cell. During operation of the electrochemical cell continued free circulation of the electrolyte, typically referred to as the “working electrolyte,” is required for rapid charge transfer. Such charge transfer occurs during charge and discharge cycling of the capacitor.
During formation, a power source capable of delivering a constant electrical current of about 100 mA per anode and a constant electrical potential of several hundred volts is connected to the anode slug that is immersed in the electrolyte. Electrical energy as high as 10 Watts per anode may be dissipated as heat and local differences in applied electrical potential may be encountered.
Regardless of the process by which the valve metal powder was processed, pressed and sintered valve metal powder structures, and specifically tantalum and niobium pellets, are typically anodized by the controlled application of formation potential and electrical current while the anode is immersed in the formation electrolyte. A typical formation electrolyte consists of ethylene glycol or polyethylene glycol, de-ionized water and H3PO4 and has a conductivity anywhere between 50 μS/cm (read: micro-Siemens per cm) to about 20,000 μS/cm at 40° C. Conventional practice has been to form the anodically polarized valve metal to a target formation potential with a constant electrical current flowing through the anode-electrolyte system. Typically, stainless steel cathodes are used with the glycol-containing electrolytes.
In the context of medical devices, capacitors are typically charged and discharged rapidly for delivery of low voltage or high voltage stimuli. Upon or during detection of a potentially lethal arrhythmia, suitable electrical transformer circuitry charges one or more high voltage capacitors using a low voltage battery as a charge source. Then, at an appropriate time the energy stored in the capacitor(s) discharges through a pair of electrodes disposed in or near a patient's heart. The magnitude of the discharged energy is used to terminate the arrhythmia and commence organized cardiac activity. Medical devices that deliver cardioversion and/or defibrillation therapy include automated external defibrillators (AEDs) and implantable cardioverter-defibrillators (ICDs). For purposes of the present invention, an ICD is understood to encompass all such IMDs having at least high voltage cardioversion or defibrillation capabilities. In most all IMDs, energy, volume, thickness and mass are critical features. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the effective cardioversion/defibrillation therapy have historically been relatively bulky and expensive.